The present invention generally relates to seat belt retractors and more particularly the class of retractors designated as energy absorbing retractors.
The classic type of seat belt retractor comprises a frame with a spool rotationally mounted upon the frame. The spool will typically include one or more lock wheels each having a plurality of teeth which are engaged by a corresponding lock pawl which is typically rotationally mounted to the frame and movable from a disengaged position to an engaged position in locking engagement with a tooth of the lock wheel. In another retractor the lock pawls are replaced by locking formations (or teeth) positioned in the frame and the spool is permitted to rotate or translate into locking engagement with these locking formations. This type of conventional seat belt retractor is known as a frame locking retractor. In either of these retractors once the spool is locked, further protraction of the seat belt is prohibited and the forward motion of the occupant is also generally restricted. As is known in the art, the seat belt is typically wound about the spool. One skilled in the art will appreciate that all forward motion of the occupant will not be stopped since as the occupant loads the locked retractor, the seat belt will be stressed and will stretch. The characteristic moduli of elasticity of a typical woven seat belt is between 8% and 16%.
In an energy absorbing retractor the spool is initially locked during the initial moments of an accident by means of a locking pawl activated by a vehicle sensor or a web sensor. Subsequently, as the accident progresses, momentum is transferred to the occupant and the occupant will tend to move forward against the seat belt and load the now locked retractor (as would happen with a conventional seat belt retractor). However, with an energy absorbing retractor the spool and its associated mechanisms are permitted to move and the seat belt is controllably permitted to protract in response to the load imparted to the seat belt by the occupant. The forward motion of the occupant is restricted by a reaction force or torque generated within the retractor. In this way the protraction of the seat belt and the forward motion of the occupant are controlled. Energy absorbing seat belt retractors often employ a deformable member such as a crushable bushing or a torsion bar. In either case, the bushing is crushed or the torsion bar rotated beyond its elastic limit into its plastic range or zone of operation to generate the desired (theoretically constant) reaction torque which acts against the forces imparted to the seat belt by the moving occupant and the torque transferred to the retractor spool.
The ultimate goal of an energy absorbing retractor is to generate a generally constant reaction force to oppose the forward motion of the occupant and to be able to generate this constant force during the entire time that the seat belt is loaded by the occupant. In theory this can be achieved by utilizing a material that effectively does not have an elastic zone and by always operating the crush bushing or the torsion bar in their constant plastic zone.
In prior art torsion bar seat belt retractors, one end of the torsion bar is fixedly attached to a lock wheel and the other end is fixedly fixed to the spool of the retractor. During an accident the lock wheel is prevented from rotating by interposing a lock dog or lock pawl within the teeth of the lock wheel. As the seat belt is loaded by the occupant, the spool will tend to rotate in opposition to the reaction torque generated within the torsion bar, as the torsion bar is twisted. The generated reaction torque depends upon the amount that the torsion bar is rotated or twisted as well as upon the physical characteristics of the torsion bar.
More specifically, the reaction torque generated by a torsion bar will vary depending upon whether the torsion bar is in its elastic, transition or plastic zones or ranges. As mentioned, in an ideal torsion bar, the elastic range is characterized by a steep (preferably infinitely steep slope or deflection curve) and the plastic range is characterized by a perfectly constant torque deflection region having a sharp transition from the elastic region. As such, once a first end of the torsion bar is locked and the spool loaded, the torsion bar will immediately make a transition from its elastic range into the plastic range of operation such that a constant reaction force is generated by the retractor as the seat belt is protracted.
It is an object of the present invention to provide a torsion bar which reduces the amount of rotation needed to generate a torque which approaches the idealized constant level, plastic reaction torque. A further object of the invention is to provide a torsion bar, energy absorbing seat belt retractor which generates a reaction torque force consistent with the torque levels achievable in the plastic zone when the seat belt is loaded by the occupant. Another object of the present invention is to provide a torsion bar seat belt retractor in which the torsion bar has a reduced transition zone. It is another object of the present invention to provide a torsion bar energy absorbing seat belt retractor in which a torsion bar having a circular cross section is pre-torqued beyond its yield torque, then released, or a torsion bar with an annular cross section that has not been pre-torqued to achieve the above objects.
Accordingly, the invention comprises: an energy absorbing seat belt retractor comprising: a frame; a torsion bar means rotationally supported relative to the frame for generating a predetermined reaction torque as it is twisted, the torsion bar characterized by an elastic deformation zone and a sharp onset into a plastic deformation zone; a spool operatively connected to rotate with the torsion bar; lock means, adaptable during a vehicle accident and operatively connected to a first portion of the torsion bar for, at least, temporarily stopping the torsion bar and the spool from rotating; the spool having a seat belt positioned thereon, wherein with the lock means activated to prevent the first portion of the torsion bar from rotating and with a load applied to the seat belt, the spool and the torsion bar are rotatable in a direction of seat belt protraction opposed by the reaction force generated by the torsion bar as it twists. In one embodiment of the invention the torsion bar is pre-torqued to create a permanent deformation therein while in another embodiment a torsion bar having an annular construction is used.
Many other objects and purposes of the invention will be clear from the following detailed description of the drawings.